I've been considering the same thing, my 472 powered '55 Caddy got 12-13 on a recent road trip, if I can get that up around 15 or better I'd be tickled to death. It's got a TH400 and a 3:36 gear... I was thinking of dropping to about a 2:73 gear... I know that 472's got plenty of torque to pull it.
Cadillacs used really low (number) gears with the big motors....think 2.20 to 2.4. Lincolns and big Fords ran 2.47 gears in the mid-late 70s. With a 2.73 you'll still have a little bit of acceleration, too.
My 1949 cadillac Coupe Deville has a 500 Cad engine, the rear gear is a 2.50 9" Ford out of a 1976 Ford Tbird. Has great driveability, will smoke tires as long as you want to & get 16-18 MPG on highway at 65-70 MPH. It has a long tailshaft TH400 trans.
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